A long standing problem with respect to compression molding of fiber-containing thermosetting resin has been surface finish of large structural articles. A particularly difficult problem with respect to production of such articles has been removal of surface irregularities commonly referred to as "sink."
U.S. Pat. No. 3,903,343 relates to sink reduction by molding a compound comprising long and short fibers. The short fibers flow into ribs. The weight ratio of short to long fibers is greater than one.
The article "Sink Reduction Techniques for SMC Molding" by Amplthor (33rd Annual Technical Conference, 1978, Reinforced Plastic/Composites Institute, The Society of Plastics Industry, Inc.) describes how certain parameters affect sink. The article illustrates a dual pressure molding cycle as a means to reduce sink. Also, the article describes other parameters that may reduce sink.
The article "SMC-Sink Mechanisms and Techniques of Minimizing Sink" (Jutte, (paper 730171) SAE, Jan. 12, 1973) also describes a variety of parameters including rib design, that may reduce sink.